The present invention generally relates to vapor cycle compressors and, more particularly, to a low cost two-stage vapor cycle compressor and a method for operating an electrically driven two-stage vapor cycle compressor.
Vapor compression refrigeration is a refrigeration method that is widely used for air-conditioning spaces, for example, public spaces such as private and public buildings, automobiles, and aircraft cabins, or for domestic or commercial refrigerators and other commercial and industrial services. Vapor-compression refrigerant systems typically circulate a liquid refrigerant as a medium that absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Vapor-compression refrigerant systems typically include a compressor, a condenser, a throttle or expansion valve, and an evaporator. The circulating refrigerant enters the compressor in a thermodynamic state known as superheated vapor, which has a low pressure and a low temperature, and is compressed to a higher pressure, resulting in a higher temperature as well. The hot vapor is routed through a condenser where it is cooled and condensed into a liquid. The liquid refrigerant is routed through the expansion valve to the evaporator, where the refrigerant absorbs and removes heat from air circulating through the evaporator and goes over into the superheated vapor state. To complete the refrigeration cycle, the refrigerant in vapor form is routed back to the compressor. Consequently, the main purpose of the compressor is to boost the pressure of the refrigerant in vapor form so that the refrigerant cycle can be completed.
A typical two-stage vapor cycle compressor includes two impellers to realize two stages of compression. Industries, and especially the aerospace industry, typically strive for vapor cycle compressors that have a high reliability and long life span, that have a compact size, are easy to assemble, and can be manufactured at a low cost while operating highly efficiently. U.S. Pat. No. 6,564,560, for example, utilizes ceramic roller element bearings to achieve an oil-free liquid chiller. Still, the roller element bearings have to be actively lubricated by liquid refrigerant.
U.S. Pat. No. 5,857,348, for example, utilizes non-lubricated radial bearings, such as magnetic or foil gas bearings cooled with refrigerant in vapor form, as journal bearings. First and second stage impellers are mounted on opposite ends of a drive shaft driven by a high-speed brushless DC (continuous current) permanent magnet motor. This layout may not allow a compact design of the compressor. The arrangement of the compressor components on the drive shaft and the use of return channels and guide vanes may not enable the most efficient cooling method for the air bearings and the motor but may increase the number of parts used in the assembly of the compressor.
U.S. Pat. No. 6,997,686, for example, teaches a two-stage compressor including a first impeller and a second impeller connected in series by a transition pipe and using a low-pressure refrigerant, such as R134a. Foil gas bearings are used in combination with an induction motor running at high speeds. An encoder disc is included to sense the rotational speed of the rotating assembly of the compressor. The compressor housing includes a separate cooling inlet and outlet for circulating liquid refrigerant in an inner cooling jacket. O-rings are used to seal the cooling jacket within the compressor housing.
As can be seen, there is a need for a two-stage vapor cycle compressor that has a simple design including a reduced number of parts and interfaces compared to prior art compressors and that can be manufactured at a relatively low cost by taking advantage of modern high volume production techniques. Furthermore, there is a need for a method that optimizes the flow cooling the bearings and the motor to increase the efficiency of the compressor compared to prior art compressors.